Surgical relief of left ventricular obstruction by left ventricular myotomy/myectomy or mitral valve replacement is a therapeutic option in patients with hypertrophic cardiomyopathy who are severely symptomatic and refractory to medical management. To determine the effects of surgical relief of left ventricular outflow obstruction in patients with hypertrophic cardiomyopathy, 5 patients were studied at rest and during atrial pacing before and after operation (septal myectomy in 4 and mitral valve replacement in 1). Coronary flow to the anterior left ventricular septum and free wall, the site of maximum hypertrophy in these patients, was assessed by thermodilution. In all 5 patients there was successful relief of rest left ventricular outflow tract gradient from a preoperative gradient of 85\plus/minus\36mmHg to 4 plus/minus8mmHg postoperatively. Surgical relief of left ventricular outflow tract obstruction reduced left ventricular systolic pressure, coronary flow, and myocardial oxygen consumption at rest and during pacing, lowered left ventricular end-diastolic pressure following pacing, and improved angina threshold and metabolic evidence of ischemia. These results demonstrate the importance of left ventricular outflow tract gradients in hypertrophic cardiomyopathy.